Low toxicity, high efficiency: ZnSe/ZnS quantum dots as photocatalysts
Chemistry is increasingly making use of the trick plants can do with
photosynthesis: driving chemical reactions that run poorly or do not
occur spontaneously at all with light energy. This requires suitable
photocatalysts that capture light energy and make it available for the
reaction. In the journal Angewandte Chemie, a Chinese research
team has now introduced layered core/shell quantum dots that efficiently
drive challenging organic transformations. Their low toxicity is a
particular advantage.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Quantum dots are finely dispersed nanoscopic crystals of inorganic
semiconductors. They absorb strongly in an adjustable range of the
spectrum and are easy to recycle. Until now, photocatalytic quantum dots
have been based almost exclusively on the highly toxic elements cadmium
and lead. This and their limited efficiency have been the main barriers
to their broader use.
A research team led by Kaifeng Wu (Chinese Academy of Sciences) has
now introduced novel quantum dots with very low toxicity and very high
performance. They are activated by commercially available blue LEDs—the
UV light that is usually required is not needed. The secret to their
success lies in their core/shell structure and the variable coatings
that can be used to “store” the light energy.
The quantum dots are only a few nanometers wide. Their core consists
of zinc selenide (ZnSe) and is surrounded by a thin shell made of zinc
sulfide (ZnS). Blue light raises the zinc selenide to an excited state
in which it can easily give up electrons. The shell prevents the
electrons from immediately being captured by so-called defects.
The team equipped the surface of the shell with special benzophenone
ligands that “suck up” the electrons from the quantum dots, store them,
and make them available for organic reactions. For example, the team was
able to carry out reductive dehalogenations of aryl chlorides and
additive-free polymerizations of acrylates—important reactions that run
poorly or not at all by conventional photocatalysts.
A second version was made by coating the surface with biphenyl
ligands that can directly absorb energy from excited quantum dots. This
brings them into a long-lived, highly energetic triplet state. The
triplet energy “stored” in this way can be transferred to specific
organic molecules, which then also enter a triplet state. In this state,
they can undergo chemical reactions that are not possible in their
ground state. As a demonstration, the team carried out [2+2]
homo-cycloadditions of styrene and cycloadditions of carbonyls with
alkenes. These produce four-membered rings (cyclobutanes or oxetanes,
respectively), which are substances that are important starting
materials in areas such as pharmaceutical development.
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About the Author
Dr Kaifeng Wu is a
principal investigator at Dalian Institute of Chemical Physics, Chinese
Academy of Sciences. His research focuses on time-resolved spectroscopy
of low-dimensional materials for emerging energy and quantum
technologies. He is the recipient of many awards including the Victor K.
LaMer award from the American Chemical Society and the Future of
Chemical Physics award from the American Physical Society.
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